1. Field of the Invention
The present invention relates to a device for suppressing common-mode radiation, and more particularly, to a device for suppressing common-mode radiation for a cable conductor.
2. Description of the Prior Art
With the rapid development of technologies, the operation speed and clock frequency of high-speed digital circuits have been increasing, and differential microstrip lines and strip lines are widely used to transmit data at high speed. Ideally, a differential transmission line is characterized by high noise tolerance, low electromagnetic radiation and low crosstalk effect. However, the differential mode circuit configuration may cause a noise voltage source or a noise current source on the circuit board. For example, a time-varying magnetic field generated by a common mode current loop of the differential mode circuit is coupled to other potential paths to generate an induced electromotive force (emf). The emf, the extended ground plane and the connected cable conductor altogether form an antenna, thereby causing common-mode radiation. For example, a capacitance between a differential mode transmission line, the extended ground plane and the connected cable conductor generates a displacement current and causes common-mode radiation.
Some methods have been proposed to suppress common-mode radiation caused by the connected cable conductor. In the most traditional method, a cable conductor is threaded through a ferrite bead, and when signals pass through the ferrite bead, the electromagnetic interference is reflected or absorbed and then converted to thermal energy for dissipation. However, the method is applicable only to the suppression of common-mode noise in the frequency band of a few hundred MHz and is seldom applied to suppress common-mode noise in the GHz frequency band in the cable conductor. Moreover, the ferrite bead is of great thickness and expensive.
As wireless communications are all operated in the GHz frequency band, conventional devices for suppressing common-mode noise in the cable conductor are not competent enough. Therefore, a need exists in the art for a device for suppressing common-mode radiation characterized in that it is thin, capable of effectively suppressing common-mode radiation and applicable in the GHz frequency band.